1. Field of the Invention
The present invention relates to a cold crucible induction melting apparatus, a melting method and tapping method using the same, and metals and alloys produced by using the cold crucible induction melting apparatus.
2. Description of the Related Art
A vacuum arc melting method, an electron beam melting method and a plasma arc melting method suitable for industrial scale melting of active metals such as titanium and alloys thereof have been frequently used.
However, preparing a large volume of molten metal bath is difficult since merely the surface of a molten metal is in principle heated on these melting methods. Consequently, a melt casting method, in which the bulk material is collectively melted and, after adjusting the composition in the melt, an ingot is produced by tapping, can not be used. Instead, the composition is adjusted in the bulk material itself to be melted, followed by sequential melting and solidification to produce the ingot. Although the melt-casting method described above has been sometimes used for practical melting methods of active metals such as titanium and alloys thereof, there remains a significant limitation from the viewpoint of effectively utilizing scraps having various configurations and compositions.
While use of lime-based refractory materials for the crucible have been attempted in melting active metals such as titanium and alloys thereof, the melt so vigorously reacts with the refractory material of the crucible when the temperature of the molten metal bath is raised to 1700.degree. C. or more that the oxygen content in the melt reaches to several thousands of ppm, making the metal to be out of the specification of the metallic material because the molten metal reacts with the material of the crucible to contaminate the molten metal bath itself.
Accordingly, a melting method commonly referred to a cold crucible induction melting method has been used for industrial purposes. The cold crucible induction melting method is also referred to an induction skull melting method. Therefore, the cold crucible induction melting method to be described hereinafter comprises the induction skull melting method.
A crucible assembled into an approximately cylindrical shape with conductive metal segments divided along the longitudinal direction is disposed in an induction coil to construct the cold crucible induction melting apparatus. An eddy current is generated in each segment of the crucible by the induction coil. The eddy current in the segment further induces another eddy current through the material to be melted in the crucible to generate a Joule heat for heat-melting the metallic material. This induction melting apparatus is referred as the cold crucible induction melting apparatus because the crucible is so constructed as to circulate a refrigerant such as water in order to prevent the crucible itself from being melted.
The cold crucible induction melting apparatus is placed in a vacuum chamber and is used for melting, for example, an active metal (metals such as Ti, Cr and Mg that are turned into a powder by being oxidized in the air). It can be utilized in melting of various melting raw materials for casting after adjusting their composition by collectively melting them. Therefore, the method is expected to be industrially useful for melting Ti scraps without pretreatment. The crucible should be large enough for melting the Ti alloy having a uniform composition in large scale in order to melt the Ti scrap. Accordingly, establishment of a melting technology using an industrially available size of crucibles have been desired. A melting technology available for producing an ingot with a weight of at least about 100 kg is required for practically melt-casting the Ti scrap in an industrial scale, which requires a technology that can collectively process at least about 150 kg of the melt weight for producing the foregoing scale of the ingot. Since the crucible is required to have an inner diameter of at least 400 mm for treating the melt having the volume described above, the melting technology should be available for this production scale. Once scale-up of the crucible has been realized, it is naturally expected to be adaptable for a variety of casting method including a continuous casting method. Therefore, a technology by which the melt can be tapped with a constant flow rate from the start to the end of tapping and a technology by which tapping can be halted or resumed on the way of tapping should be established.
Three tapping methods of the cold crucible induction melting apparatus as hitherto described are illustrated in FIG. 1, FIG. 2A, FIG. 2B and FIG. 2C, and FIG. 3. FIG. 1 shows a tilt-tapping method by which the melt is tapped by tilting the whole crucible. FIG. 2A-C a bottom-tapping method by which the melt is tapped from a nozzle attached at the bottom of the crucible. FIG. 3 shows a bottom-tapping method in a levitation type cold crucible.
Since the solidified skull 2 is markedly grown due to increased contact area between the melt 1 and the side wall 11 of the crucible due to tilting of the crucible unit (crucible 10 and melting coil 21 ) during tapping in the tapping method shown by the reference numeral 2a in FIG. 1, the amount of tapped melt is accordingly decreased. Therefore, it is desirable in the tilt-tapping method to tap the melt as soon as possible to shorten the residence time of the melt in the crucible while it is tilted. The overall melt should be tapped within several seconds by a momentary tilting. When tapping is once halted in this tapping method, the contact area between the melt and the wall of the crucible becomes so large while the crucible unit remains to be tilted that the solidified skull is markedly grown. Therefore, the solidified skull 2a should be melted again for resuming tapping after returning the tilted crucible to its original position, requiring much time for resuming tapping after the halt of tapping.
In the cold crucible induction melting apparatus using the conventional bottom tapping method (FIG. 2A to FIG. 2C), a melting subject is melted by allowing it to contact with the bottom and inner wall of a crucible while forming a solidified skull 2 on the surface of the melting subject, followed by allowing a nozzle tap 3 disposed at the top of a tapping nozzle 13 to melt with a melting coil (operated at, for example, 1800 kW and 1 kHz) 21 and a tapping coil (operated at, for example, 400 kW and 4 kHz) 22 for tapping. Since the tapping rate in this method is proportional to the square root of the height from the nozzle hole to the surface of the melt (the height of melt surface), the tapping rate will be decreased as tapping is proceeded to reduce the height of the melt surface.
Although tapping becomes easy by expanding the diameter of the nozzle, it becomes impossible to halt tapping on its way when the nozzle diameter is too large. Even if halting of tapping is possible by forming a solid phase (the reference numeral 2b in FIG. 2C) by cooling, the heat transfer rate from the solid phase in the nozzle to the tapping nozzle made of water-cooled copper is far more larger than the heat transfer rate from the nozzle tap 3, which is initially inserted without making a contact with the inner face of the nozzle, to the water-cooled copper nozzle. In addition, the solid phase located upward of the nozzle is positioned apart from the melting coil 21 besides the induction current from the tapping coil 22 can not be sufficiently generated in the solid phase. Accordingly, melting of the solid phase is so difficult that resume of tapping is considered to be impossible.
Control of the tapping nozzle becomes easy, continuous casting is readily carried out and halting of tapping on its way is also easy along with being favorable for pouring the melt into a mold little by little when the diameter of the tapping nozzle is small. However, it is a problem that tapping becomes impossible because the area where the solidified skull makes a contact with the inner surface of the nozzle and the bottom of the crucible becomes rather wide, the amount of heat transfer to the water-cooled copper crucible and the tapping nozzle is increased and the solidified skull in the tapping nozzle can not be completely melted. Accordingly, it was necessary in tapping from the tapping nozzle with a small diameter to supply a large electric power exceeding the power required for melting since the heat supplied to the solidified skull from the melting coil, or the energy density (the amount of coil current), has an upper limit. However, the melt in the crucible vigorously undulates when an electric power more than necessary is supplied to the cold crucible melting furnace, causing a decrease of power efficiency while allowing the melt to splash out of the crucible.
In the levitation type cold crucible induction melting apparatus (FIG. 3), a high frequency coil (operating at, for example, 1000 kW and 30 kHz) is used for the upper melting induction coil 25 and a relatively low frequency coil (operating at, for example, 1000 kW and 3 kHz) as compared with the induction coil described above is used in order to endow the lower induction coil 26 with a large levitation force for melting the metal while allowing it to float. Since it is difficult, however, in the levitation type cold crucible induction melting apparatus to float a large quantity of molten metal, a melting apparatus merely available for about 50 kg of the melting subject is currently operating and the apparatus has not been applied for industrial scale production. Since two contradictory functions of levitation function and tapping function are expected for the lower induction coil 26, it is quite difficult to stably hold the levitation melt during tapping to make control of the tapping rate difficult in this method. It is pointed out that output control of the upper induction coil 25 and lower induction coil 26 for resuming levitation, melting and tapping is very difficult once tapping has been halted.
As hitherto described, it is impossible to efficiently control the tapping rate and to tap, halt and resume tapping within a short period of time. Degree of freedom in casting is small, or fractional casting and tapping into a continuous casting machine are difficult, thus currently narrowing the applicable range of the methods.